A quantitative autoradiographic method for the determination of local rates of protein synthesis in brain in vivo has been developed with L-[1-C(14)]leucine as the tracer amino acid. A four-compartment model for the behavior of leucine in brain has been analyzed, and an equation has been derived that defines the rates of amino acid incorporation into protein in terms of the time course of the plasma leucine specific activity, final tissue concentrations of C(14), and lambda-tRNA, the steady state ratio of the distribution volumes for the labeled and unlabeled leucine in the immediate precursor pool for protein synthesis. This constant, lambda tRNA, is a measure of the fraction of leucine in the precursor pool for protein synthesis that is derived from plasma; the remainder is derived from protein degradation. The value of lambda tRNA for the brain as a whole has been experimentally, determined to be 0.58. Preliminary results suggest that lambda-tRNA may vary regionally, with values ranging from 0.4 in white matter to 0.8 in grey matter. In an attempt to increase the value of lambda-tRNA toward 1.0, we have administered flooding doses of unlabeled L-leucine to rats. Under these conditions, however, free leucine levels in brain do not increase more than 4-fold. We have tried similar experiments with L-valine and have found 30-fold increases in free valine levels in brain and an increase in lambda- tRNA for valine from 0.37 under normal conditions to 0.74 under flooding conditions. These results show that the amino acid precursor pool for protein synthesis is partially derived from protein degradation; the fraction of the pool coming from protein degradation varies with each amino acid and in different brain regions. The relative size of this fraction can be reduced by flooding the brain amino acid pools with the unlabeled amino acid. Studies carried out on applications of the method that examine some basic neurobiological questions include: 1) barbiturate & ketamine anesthesia; 2) slow wave sleep; 3) normal development; 4) electrical stimulation; 5) hypothyroidism; 6) regeneration; and 7) circadian rhythms.